This invention relates to a cooling process suitable for foam molding of a synthetic resin and its cooling apparatus.
Generally, a foamed plastic product made of e.g. foamable styrene resin is obtained by the following process: firstly styrene resin granules containing and foaming agent is performed to produce foamable beads, then matured and finally further expanded in a pair of molds by the application of steam heat. If necessary, the foamed plastic product as molded thus may be dried further. The aforementioned processing operations are conventionally carried out as follows:
(1) Closing a male mold and a female mold between which is formed a cavity, PA1 (2) Preheating the pair of molds by supplying steam into chambers mounted on the exterior of the molds, (3) Opening a pair of molds a little bit (normally 2 mm to 6 mm) so that the prefoamed granules are not passable into a slight gap therebetween, PA1 (4) Filling prefoamed beads into a cavity with air using a filling gun, PA1 (5) Full-scale heating foaming and fusing the prefoamed beads and discharging at a steam pressure of 0.8 to 1.2 Kg/cm.sup.2 G after maintaining it for several seconds, Z PA1 (6) Cooling the mold by means of water-shower and/or compressed air, PA1 (7) Opening the mold for taking out the molded product. PA1 (1) To cool a certain portion of the mold to which the cooling water is not applied, a longer cooling time is required, and the use quantity of the cooling water is increased. PA1 (2) A directly cooled portion of the mold is apt to be cooled excessively. Cooling water which penetrates into a slit between a chamber and a cavity of the molds may be absorbed into a molded product or may wet its surface. Such a wet molded product must be dried before packaging and shipment. PA1 (3) As the result of the aforementioned disadvantages (1) and (2), an average mold temperature must be decreased to less than 50.degree. C., that is, less than the required temperature. For this reason, preheating may be required or the steam quantity to be used for heating may be increased. PA1 (4) Condensation generated when preheating the mold by steam and cooling water remaining on the mold surface hinder filling because the beads are bonded together by surface tension. Thus, conventional water-cooling system has a lot of defects. As a result, much of the heat capacity for fundamental heating foam molding is consumed uselessly, the molding cycle time is prolonged and productivity is degraded remarkably.
The task of this invention is to obtain a cooling process most suitable in the manufacturing process of the foamed synthetic resin product, and its cooling apparatus.
The foamable synthetic resin material of this sort is heated by steam after prefoamed beads have been filled in the cavity, then prefoamed, fused and molded to the shape of the cavity. At that time, some foamed gas pressure remains inside the molded product. When taking it out of the molds, there is the danger that the molded product may be deformed due to expansion caused by residual gas pressure. In order to avoid deformation of the molded product and to obtain a foamed product equivalent to the cavity profile, an effective cooling operation is required and brings about a remarkable effect upon quality of the molded object.
Particularly when the shape of the foamed product is complicated and its thickness is not uniform, the inconvenience is that a mold cooling operation is not carried out uniformly, consequently a decline in cooling efficiency causing a longer molding cycle and poorer productivity. Accordingly, it is a very important task to develop a suitable and efficient cooling process in the manufacturing process of this sort of foamed synthetic resin product, and an apparatus for performing the coding process.
Before describing in detail a preferred example of this invention, a conventional cooling process which is being practiced will now be described. Simultaneously, the disadvantage of the conventional art will be discussed.
In FIG. 1 there are shown a pair of conventional molds 1, 2 (a female mold 1 and a male mold 2) between which is formed a cavity 3. Suitably prefoamed beads are filled into the cavity 3 by a filling gun 4. On the respective exteriors of the molds 1, 2 there are mounted chambers 5, 6. Steam is supplied to the chambers 5, 6 by the way of pipes 7, 7. The two chambers 5, 6 are provided, at preferred portions of pipes 9 for supplying cooling water, with a plurality of shower nozzles 8, 8. The pressurized cooling water is jetted from a plurality of shower nozzles 8, 8. The female mold 1 is provided with one or more eject pins 10 by which a molded foamed object A can be taken out. Numerals 7', 7' are respective drain pipes of the two chambers 5, 6.
The cooling apparatus of the above structure adopts a water spray treatment by jetting pressurized cooling water exclusively from a plurality of shower nozzles. The molds are cooled by directing the cooling water directly on the interior of the molds 1, 2.
Further, there is a known air-cooled system in which compressed air is supplied from nozzles.
Still further, there is a very simple cooling method, that is, radiation without using water, compressed air, etc.
Conventionally, either one of the above three cooling ways, i.e. a water spray cooling system, an air-cooling system and a simple radiation system or two or more of them in combination has been used.
The above second and third cooling ways have poor cooling effect and are disadvantageous, in various ways, for foam molding of a foamable synthetic resin material for purpose of high efficiency and production.
According to the water spray cooling system, it is practically impossible to locate a plurality of shower nozzles so that the cooling water can be uniformly deposited on an overall portion of the molds of a complicated shape for molding various foamed products (e.g. packaging medium) other than a simple sheet-type foamed one. As a result, one portion of the mold is cooled directly by the cooling water, but the other portion thereof is cooled indirectly, that is, out of a direct jetting of the cooling water. Such an imbalance causes the following inconveniences and disadvantages.